The default “Multicast Group Subnet” 239.1.1.0/25

[nkhawaja1]

Hi,

I was trying to find some best practices about what subnet mask to pick for “Multicast Group Subnet”.

It is a NDFC deployed 2xVXLAN fabrics. When TRM is enabled, it asks you for “Multicast Group Subnet” and picks up the default as 239.1.1.0/25

Would it be ok to use  239.1.0.0/16, just wanted to make sure it won’t impact anything and is in accordance with the best practices.

Thanks,

[M02@rt37]

Hello @nkhawaja1,

Firstly, it's important to note that multicast group addresses fall within a specific range of IP addresses, which is defined as 224.0.0.0/4. This range is reserved for multicast traffic and should not be used for unicast traffic.

The choice of subnet within the multicast address range depends on the specific requirements of your network. In general, it's best to use a subnet that is not already in use on the network to avoid any conflicts.

The default subnet that is suggested by the TRM tool, 239.1.1.0/25, is a valid multicast group address subnet and should work fine for most networks. However, if you have a specific reason for choosing a different subnet, such as to avoid conflicts with an existing multicast group, then you can choose a different subnet within the multicast address range.

Using 239.1.0.0/16 should also work fine as it falls within the multicast address range and is not already in use. However, it's important to ensure that any other devices or applications that will be using multicast group addresses are also configured to use the same subnet to avoid any conflicts.

[mr.richardb]

The Multicast Group Subnet is as the description of the filed said " A multicast group IP from this pool is used for BUM traffic for each overlay network". This means that NDFC will assign a multicast group address  for BUM from this range whenever you create a network. 

You either can map multiple overlay networks to the same group address or assign separate group address to each overlay network. I think the second is better for enterprise fabrics (more clear from tshoot POV) but you need to take into consideration that there is a limit how many group address can you platform  host. you should check the sw scalability guide.

https://www.cisco.com/c/en/us/td/docs/dcn/nx-os/nexus9000/102x/configuration/scalability/cisco-nexus-9000-series-nx-os-verified-scalability-guide-1022.html

Underlay multicast groups

Nexus 9300-EX/FX/FX2/FX3/GX switches, and the Nexus 9700-EX/FX and N9K-X9716D-GX line cards

512

It depends of the size of you implementation But for smaller fabric the /25 should be sufficient. (that is 128 individual networks if you stick with the 1 group address to 1 overlay network)

Generally use "something" from the 239.0.0.0/8 range as that is the Administratively scoped address range which is for private use within an organization.

Regards,

Richard